Strain field reconstruction method and system based on spirally wound optical fiber DAS

Abstract

The invention discloses a strain field reconstruction method and system based on a spiral winding optical fiber DAS, and relates to the technical field of unconventional oil and gas reservoirs, and the method comprises the steps: enabling a detection optical fiber of a sensor to be periodically wound on a shaft of a target underground strain field; two sampling points are respectively arranged on the first optical fiber section and the third optical fiber section at a first angle sampling interval, and four sampling points are respectively arranged on the second optical fiber section and the fourth optical fiber section at a second angle sampling interval; obtaining the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field; and reconstructing an original strain component of the target underground strain field based on the DAS response of each sampling point. According to the method, the technical problem that in the prior art, calculation of Hessian matrix inverse in strain field component reconstruction is very difficult is solved.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas reservoirs, and in particular to a strain field reconstruction method and system based on spirally wound optical fiber DAS. Background Art

[0002] For unconventional oil and gas reservoirs, distributed optical fiber sensing (DAS) technology has become a popular seismic acquisition tool due to its advantages such as easy deployment, wide bandwidth range and high sampling density. As a new acquisition tool, DAS has developed rapidly in theoretical development and industrial application. Due to the inhomogeneity of the optical fiber material medium, DAS can generate strain response using the phase signal of Rayleigh scattered waves. At present, DAS technology has been widely used in seismic monitoring, vertical seismic profiling (VSP) reservoir prediction, near-surface exploration and full waveform inversion.

[0003] Due to the low cost of optical fiber materials, researchers can deploy long-distance arrays and obtain DAS acquisition data with wide bandwidth and high density. In addition, DAS optical fiber is more feasible than detectors in downhole environments and can be extended to more complex formations. These advantages show that DAS technology has great potential application value in seismic data processing and interpretation. However, vertical or horizontal optical fibers are only sensitive to uniaxial strain, which means that DAS can only obtain single-component seismic records, which makes it difficult to apply to multi-component seismic data processing.

[0004] Through research, it is found that spirally wound optical fiber can receive projection signals from all seismic strain field components and reconstruct these strain components for multi-component data processing. However, the projection matrix of traditional spirally wound optical fiber is a singular matrix, and there is a technical problem that the calculation of the inverse of the Hessian matrix in the reconstruction of strain field components is very difficult. Summary of the invention

[0005] The purpose of the present invention is to provide a strain field reconstruction method and system based on spirally wound optical fiber DAS in order to solve at least one of the above technical problems.

[0006] In a first aspect, an embodiment of the present invention provides a strain field reconstruction method based on a spirally wound optical fiber DAS, which is applied to a spirally wound optical fiber distributed acoustic sensor; the method comprises: periodically winding the detection optical fiber of the sensor on the wellbore of the target underground strain field; wherein the detection optical fiber in each winding cycle comprises a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle; two sampling points are respectively set at a first angle sampling interval on the first optical fiber segment and the third optical fiber segment, and four sampling points are respectively set at a second angle sampling interval on the second optical fiber segment and the fourth optical fiber segment; the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field is obtained; and the original strain component of the target underground strain field is reconstructed based on the DAS response of each sampling point.

[0007] Further, the winding period of the detection optical fiber includes a 2π period, the first angle sampling interval includes π / 8, and the second angle sampling interval includes 3π / 16.

[0008] Furthermore, based on the DAS response of each sampling point, the original strain component of the target underground strain field is reconstructed, including: based on the sampling point distribution of the sensor, determining a projection matrix of the mapping relationship between the DAS response of each sampling point and the original strain component of the target underground strain field; based on the projection matrix and the DAS response of each sampling point, reconstructing the original strain component.

[0009] Furthermore, the mapping relationship includes:

[0010] ε=(G T G)- 1 G T ε d

[0011] Where ε=[ε xx ε yy ε zz ε yz ε xz ε xy ] T is the strain vector containing the original strain components, εd is the DAS response;

[0012] G is the projection matrix, which is a 6×6 matrix, and the i-th row vector is

[0013]

[0014]

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021] Wherein, i is the serial number of the sampling point, for the sampling points of the first optical fiber segment and the second optical fiber segment:

[0023] For the sampling points of the third optical fiber segment and the fourth optical fiber segment:

[0024]

[0025] α1 is the first winding angle, α2 is the second winding angle, θ s1 is the first angle sampling interval, θ s2 is the second angle sampling interval, M is a preset non-negative integer, GL is the gauge length, and r is the winding radius of the wellbore.

[0026] In a second aspect, an embodiment of the present invention further provides a strain field reconstruction system based on a spirally wound optical fiber DAS, which is applied to a spirally wound optical fiber distributed acoustic sensor; comprising: a winding module, a setting module, an acquisition module and a reconstruction module; wherein the winding module is used to periodically wind the detection optical fiber of the sensor on the wellbore of the target underground strain field; wherein the detection optical fiber in each winding cycle comprises a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle; the setting module is used to set two sampling points at a first angle sampling interval on the first optical fiber segment and the third optical fiber segment, and to set four sampling points at a second angle sampling interval on the second optical fiber segment and the fourth optical fiber segment; the acquisition module is used to obtain the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field; the reconstruction module is used to reconstruct the original strain component of the target underground strain field based on the DAS response of each sampling point.

[0027] Furthermore, the reconstruction module is also used to: determine a projection matrix of a mapping relationship between a DAS response of each sampling point and an original strain component of the target underground strain field based on the sampling point distribution of the sensor; and reconstruct the original strain component based on the projection matrix and the DAS response of each sampling point.

[0028] In a third aspect, an embodiment of the present invention further provides an electronic device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method described in the first aspect when executing the computer program.

[0029] In a fourth aspect, an embodiment of the present invention further provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and when the computer instructions are executed by a processor, the method described in the first aspect is implemented.

[0030] The present invention provides a strain field reconstruction method and system based on a spirally wound optical fiber DAS, which utilizes an irregular variable angle winding scheme and a spiral optical fiber DAS response to reconstruct the underground strain field, so that the condition number of the projection matrix is ​​low enough to perfectly restore the original strain field, thereby alleviating the technical problem in the prior art that the calculation of the inverse of the Hessian matrix in the reconstruction of the strain field components is very difficult. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the implementation methods of the present application or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 A flow chart of a strain field reconstruction method based on a spirally wound optical fiber DAS provided in an embodiment of the present invention;

[0033] Figure 2 A schematic diagram of a fiber winding model of a conventional helical fiber DAS provided in an embodiment of the present invention;

[0034] Figure 3 A schematic diagram of a condition number and rank curve of a projection matrix provided in an embodiment of the present invention;

[0035] Figure 4 A schematic diagram of an irregular variable-angle spiral optical fiber winding model provided by an embodiment of the present invention;

[0036] Figure 5 A schematic diagram of a VTI layered model inserted into a vertical wellbore provided by an embodiment of the present invention;

[0037] Figure 6 The precision strain rate recording diagram of the VTI layered model with a length of 2 seconds provided in the embodiment of the present invention;

[0038] Figure 7 A strain rate record diagram of a VTI layered model reconstructed with a gauge length of 1.25 meters provided in an embodiment of the present invention;

[0039] Figure 8 A strain rate record diagram of a VTI layered model reconstructed with a gauge length of 4.85 meters provided in an embodiment of the present invention;

[0040] Fig. 9 A comparison diagram of single-channel data extracted from original data and DAS-reconstructed strain rate records provided in an embodiment of the present invention;

[0041] Fig.10 A strain rate record diagram of a VTI layered model with a DAS data amplitude with 3% random noise provided by an embodiment of the present invention;

[0042] Fig.11 A random noise map extracted from the difference between a reconstructed strain rate record and an original record of a VTI layered model provided in an embodiment of the present invention;

[0043] Fig.12A schematic diagram of a strain field reconstruction system based on a spirally wound optical fiber DAS provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0044] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0045] Embodiment 1

[0046] Figure 1 1 is a flow chart of a strain field reconstruction method based on a spirally wound optical fiber DAS according to an embodiment of the present invention, and the method is applied to a spirally wound optical fiber distributed acoustic sensor. Figure 1 As shown, the method specifically comprises the following steps:

[0047] Step S102, periodically winding the detection optical fiber of the sensor on the wellbore of the target underground strain field; wherein the detection optical fiber in each winding cycle includes a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle.

[0048] Step S104: two sampling points are set at the first optical fiber segment and the third optical fiber segment at the first angular sampling interval, and four sampling points are set at the second optical fiber segment and the fourth optical fiber segment at the second angular sampling interval.

[0049] Preferably, the winding period of the detection optical fiber includes a 2π period, the first angle sampling interval includes π / 8, and the second angle sampling interval includes 3π / 16.

[0050] Step S106, obtaining the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field.

[0051] Step S108: reconstructing the original strain components of the target underground strain field based on the DAS response of each sampling point.

[0052] Specifically, step S108 includes the following steps:

[0053] Step S1081, based on the sampling point distribution of the sensor, determining a projection matrix of a mapping relationship between a DAS response of each sampling point and an original strain component of a target underground strain field;

[0054] Step S1082, reconstructing the original strain component based on the projection matrix and the DAS response of each sampling point.

[0055] The mapping relationships include:

[0056] ε=(G T G) -1 G T ε d

[0057] Where ε=[ε xx ε yy ε zz ε yz ε xz ε xy ] T is the strain vector containing the original strain components, ε d For DAS response;

[0058] G is the projection matrix, which is a 6×6 matrix, and the i-th row vector is

[0059]

[0060]

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0067]

[0068] Where i is the serial number of the sampling point, for the sampling points of the first fiber segment and the second fiber segment:

[0069]

[0070] For the sampling points of the third and fourth fiber segments:

[0071]

[0072] α1 is the first winding angle, α2 is the second winding angle, θ s1is the first angle sampling interval, θ s2 is the second angle sampling interval, M is a preset non-negative integer, GL is the gauge length, and r is the winding radius of the wellbore.

[0073] In order to illustrate a complete technical solution of a strain field reconstruction method based on a helically wound optical fiber DAS provided by an embodiment of the present invention, the basic mathematical principle of the helically wound optical fiber DAS in the prior art is firstly described as follows:

[0074] Figure 2 FIG. 1 is a schematic diagram of a fiber winding model of a conventional spiral fiber DAS provided according to an embodiment of the present invention. Figure 2 As shown, (a) is the winding scheme of the optical fiber along the wellbore, and its winding angle is a fixed angle; (b) is a side expansion schematic diagram of (a), and (c) is a line segment view of the spiral optical fiber AA' in the rotation angle domain. Among them, the cylinder represents the wellbore, the green line AA' represents the spiral optical fiber, point A is the starting point in a 2π winding cycle, A' is the end point, and the green dots represent six sampling points uniformly detected in the spiral optical fiber AA', with an angle sampling interval of θ s =π / 3, P is an arbitrary point on line AA', B is the projection of point P on the base circle, 00' is the central axis of the cylinder, Tp represents the tangent vector of point P, r is the winding radius of the cylinder, θ is the rotation angle, α is the winding angle, h represents the height PB of point P, l is the length of AP, and is also the coordinate variable in line AA'. It should be noted that in Figures (a) and (b), AA' is measured by its length, and in Figure (c) it is measured by its rotation angle.

[0075] Specifically, the DAS response of a point on the helically wound fiber is the projection of the seismic strain component along the fiber tangential direction. It can be expressed as a rotation from the global coordinate system (x, y, z) to the local coordinate system (ε, m, n) as follows:

[0076] ε0=MεM T (1)

[0077] Where ε0 and ε are the rotation strain tensor and the original strain tensor respectively, and M is the rotation matrix which can be written as:

[0078]

[0079] Among them, M ij (i=t,m,n;j=t,p,q) represents the cosine of the angle between the two coordinate axes. Since the optical fiber is only sensitive to the tangential direction, the t-axis in the local coordinate system is parallel to the tangential vector of the helical fiber. At the same time, it is assumed that the seismic wavelength is large enough to keep the strain field constant over the measurement length. After that, the DAS response of the helically wound fiber ε d It can be expressed as:

[0080] ε d =G xx ε xx +G yy ε yy +G zz ε zz +G yz ε yz +G xz ε xz +G xy ε xy (3)

[0081] and:

[0082]

[0083] Among them, ε xx , ε yy and ε zz is the normal strain component, ε yz , ε xz and ε xy is the shear strain component. s p represents the location of the sampling point in the helical fiber. l represents the coordinate variable in the helical fiber. GL is the gauge length. Formulas (3) and (4) show that the DAS response is the average value of the original strain projection on the gauge length.

[0084] Specifically, in order to obtain M in formula (4) tx 、M ty and M tz , use Figure 2 The tangent vector of any point on the helical fiber can be derived from a 2π winding cycle in the helical fiber winding scheme and its unfolded side view. Figure 2 As shown in Figure (a), A and A' are the starting point and end point of the winding optical fiber, r is the winding radius along the wellbore, α and θ are the winding angle and rotation angle respectively, P is an arbitrary point in the spiral optical fiber, T p is the tangent vector at point P. In the side view ( Figure 2 (b) in the figure), define the height of point P as h and the distance AP = l (l is the one-dimensional coordinate variable on the line segment AA'). According to the geometric relationship, we can get:

[0085]

[0086] Formula (5) is an important conversion relationship between the length l of the helical fiber and its corresponding rotation angle θ. The value range of θ is [0,2π] (θ=0 corresponds to point A, θ=2π corresponds to point A'), and the maximum l max=2πr / cosα. According to formula (5), the curve equation of the spiral fiber to point P can be expressed as:

[0087]

[0088]

[0089] Where P x , P y and P z is the coordinate of point P. Formula (6) and Formula (7) show that both l and θ can be used as independent variables of the AA' curve function. However, since Formula (6) and Formula (7) mainly contain trigonometric functions, θ is needed to describe the AA' curve. Therefore, the unit tangent vector T p It can be expressed as:

[0090]

[0091] By setting the basis vectors x = (1, 0, 0), y = (0, 1, 0) and z = (0, 0, 1) in the global coordinate system, the projection coefficients M can be calculated tx 、M ty and M tz , as shown in formula (9):

[0092]

[0093] Then, substituting formula (9) into formula (4) and replacing l with θ, we get the following result:

[0094]

[0095] Among them, θ p =s p cosα / r is the angular position corresponding to the sampling point. θ g =GL / 2·cosα / r represents the angle related to the gauge length. A0=r / (GL·cosα) is the weighting coefficient. By substituting formula (10) into formula (3), the DAS response of a single point in the helical fiber can be obtained.

[0096] Formula (3) and Formula (10) represent the single DAS response point of the helical fiber. However, in order to reconstruct the original strain field component ε in Formula (3), xx ,ε yy ,ε zz ,ε yz ,ε xz ,ε xy , at least six DAS responses are required d For example, Figure 2The conventional winding scheme shown in FIG. 1 can evenly divide the spiral optical fiber into six sections and detect seismic signals at the midpoint of each section. Figure 2 For simplicity, it is assumed that six DAS response points are sampled in one 2π cycle (in the following, it is extended to an arbitrary number of winding cycles for sampling of six points). Figure 2 The projection matrix coefficients for the winding scheme shown are expressed as:

[0097]

[0098] Where i∈[1,6] represents the sequence number of the six sampling points (from Figure 2 (c) from left to right), θ s is the sampling angle interval, which is equal to π / 3 in the winding scheme. The spatial sampling interval is s = rθ s / cosα+2Nπr / cosα. N is a non-negative integer. When N=0, it means that six sampling points are extracted in a 2π winding cycle. θ b =θ s / 2 represents the angular position of the starting sampling point. Substituting formula (11) into formula (3), the vector form of the DAS response is obtained:

[0099] ε d =Gε (12)

[0100] Where ε=[ε xx ε yy ε zz ε yz ε xz ε xy ] T is the strain vector to be constructed, ε d is a column vector containing the six different DAS response values. G is a 6×6 matrix with each row having the same form Once a six-point DAS record within the seismic wavelength is obtained, a least squares solution to the raw strain field can be constructed:

[0101] ε=(G T G) -1 G T ε d (13)

[0102] Formula (13) is used to reconstruct the six components of the original strain field. The quality of the reconstructed strain field depends on the projection matrix G, which can be verified by its rank and condition number. According to formula (11), the matrix G is composed of the angular sampling interval θ s , winding angle α and gauge length GL. s=π / 3 remains unchanged. For the winding angle α, the present invention provides two test values ​​of 30° and 60°. The gauge length ranges from 0.1 meters to 10 meters. Substituting these parameters into formula (11), the condition number and rank curve of the projection matrix G can be calculated as follows: Figure 3 Specifically, Figure 3 (a) shows the condition number when the winding angle α = π / 6, (c) shows the condition number when the winding angle α = π / 6, (b) shows the rank when the winding angle α = π / 3, and (d) shows the rank when the winding angle α = π / 3. The minimum values ​​of the curves in (a) and (b) are 1.97×10 16 and 2.98×10 16 .

[0103] Specifically, Figure 3 As shown, under the above parameters, a very high matrix condition number is shown, and its minimum value even exceeds 10 16 . And its rank is 5, which indicates that the projection matrix is ​​singularly ill-conditioned. Therefore, use Figure 2 The optical fiber winding scheme in the prior art shown has difficulty in calculating the inverse matrix of the matrix G to reconstruct the seismic strain components.

[0104] In order to solve the ill-conditioned matrix problem in the traditional winding method, the present invention proposes an irregular variable-angle spiral optical fiber winding solution. Specifically, Figure 4 FIG. 1 is a schematic diagram of an irregular variable-angle spiral optical fiber winding model provided according to an embodiment of the present invention. Figure 4 As shown, in the solution proposed by the present invention, twelve-point DAS strain records need to be accurately extracted from fixed positions of one or more 2π winding periods, and the spatial sampling interval s remains unchanged.

[0105] For the sake of simplicity, the present invention Figure 4 A design with a 2π winding period was selected. Among them, (a) is the spiral fiber winding along the wellbore, (b) is the side expansion view of (a), and (c) is the horizontal straightening view of the spiral fiber AA' in the rotation angle domain. The cylinder represents the wellbore, the line AA' represents the spiral fiber divided into four segments, namely S1, S2, S3 and S4 segments, A is the starting point in a 2π winding period, A' is the end point, and the green dots indicate that two sampling points are detected in the S1 and S3 segments respectively, and the angle sampling interval θ s1 =π / 8 and winding angle α1, the red dots indicate that four sampling points are detected in the S2 and S4 segments, respectively, and the angle sampling interval θ s2 =3π / 16 and winding angle α2, OO' is the central axis of the cylinder, and r is the winding radius of the cylinder. It should be noted that in Figures (a) and (b), AA' is measured by its length, and in Figure (c) it is measured by its rotation angle.

[0106] Specifically, Figure 4 As shown, the spiral optical fiber is divided into four sections, namely the first optical fiber section, the second optical fiber section, the third optical fiber section and the fourth optical fiber section which are connected in sequence. Figure 4 S1 in the figure) and the third fiber segment (i.e. Figure 4 In S3 in , two sampling points are extracted at the midpoint of the line segment (such as Figure 4 As shown in Figure (c) in the figure), the first winding angle is α1 (as shown in Figure 4 As shown in Figure (b), the first angle sampling interval θ s1 =π / 8 (such as Figure 4 In the second optical fiber segment (i.e. Figure 4 S2 in the figure) and the fourth fiber segment (i.e. Figure 4 In S4), four sampling points are obtained through the center of the line segment, and the second winding angle is α2 (such as Figure 4 As shown in Figure (b), the second angle sampling interval θ s2 =3π / 16 (such as Figure 4 The six sampling points in the S1 and S2 segments ([0,π]) and the S3 and S4 segments ([π,2π]) are used to reconstruct different strain fields. Figure 4 As can be seen from Figure (b) in FIG. 1 , these spiral fiber parameters also satisfy the triangular geometric relationship formula (5). Therefore, the present invention can be based on Figure 4 The curve equations of the four segments are derived from any point P on the spiral fiber in Figure (a):

[0107]

[0108] According to formula (14), the four segments S1, S2, S3 and S4 share the same P x and P y Expression, but P z The coordinates are different. Substituting formula (14) into formula (8), we can get the same tangent vector T as in formula (8): p The form of , but contains two winding angles α1 and α2. At the same time, the sampling interval s and the gauge length GL need to remain unchanged during the entire seismic signal acquisition process. According to the conversion relationship of formula (5), the sampling interval and gauge length are expressed as follows:

[0109]

[0110] Wherein, N is a non-negative integer representing the number of 2π winding cycles between two sampling points. Since the same position in different winding cycles has the same tangent vector form (Formula 8), the value of N does not affect the integral of the following formulas (11) and (16). In addition, according to formula (15), α1 and α2 need to satisfy the relationship cosα1 / cosα2=2 / 3. M is also a non-negative integer and determines the gauge length. The reason why the present invention sets GL in this form in formula (15) is that the angle associated with GL can be expressed as θ g =θ s1 / 2+2Mπorθ s2 / 2+2Mπ, which is beneficial to the integral calculation of the following formula (16). Figure 4 ) and traditional solutions ( Figure 2 ), the present invention directly substitutes the above parameters into formula (11). Taking the S1 and S2 segments as an example, the coefficients of the projection matrix G of the winding design proposed by the present invention can be obtained:

[0111]

[0112] and:

[0113]

[0114] Where i = 1, 2, 3, 4, 5, 6 are the serial numbers of the sampling points of the S1 and S2 segments. For the projection matrix calculation of the S3 and S4 segments, it is only necessary to replace Add π. To simplify the integration, formula (16) can be rewritten as:

[0115]

[0116] Formula (18) is easier to calculate than formula (16) because its integration range only includes the same winding angle. By substituting formula (18) into formula (13), the original strain component can be obtained. In order to test the projection matrix (Formula 18) rank and condition number of the irregular variable-angle spiral optical fiber solution proposed in the present invention, the parameters r=0.05m, α1=66.88°, α2=53.91°, N=1, s=0.7m (the minimum sampling interval of the DAS IO instrument "FEBUS" is 0.2m), M varies from 0 to 20, and the gauge length GL is calculated according to formula 15 and varies in the range of 0.05m to 24.05m. In an embodiment of the present invention, the condition number increases with the increase of the gauge length. This is due to the averaging effect of the gauge length. However, the maximum value of the condition number is limited to 10 4 The magnitude is much smaller than 10 of the traditional solution. 16, the minimum value is only 48.14. At the same time, the rank of the matrix remains at 6, indicating that it is a full-rank projection matrix. Therefore, the spiral fiber winding design proposed in the present invention can effectively reconstruct the original strain component. Figure 2 and Figure 4 In the present invention, the projection matrix of the spiral fiber is derived using a vertical wellbore, which is similar to the VSP (vertical seismic profile) observation system. If the spiral fiber is buried on the surface for ground seismic exploration, it is only necessary to exchange the positions of x and z in formula (18) and formula (16). That is, becomes becomes vice versa.

[0117] From the above description, it can be seen that an embodiment of the present invention provides a strain field reconstruction method based on a spirally wound optical fiber DAS, which utilizes an irregular variable angle winding scheme and a spiral optical fiber DAS response to reconstruct the underground strain field, so that the condition number of the projection matrix is ​​low enough to perfectly restore the original strain field, thereby alleviating the technical problem in the prior art that the calculation of the inverse of the Hessian matrix in the reconstruction of the strain field components is very difficult.

[0118] Embodiment 2

[0119] The embodiment of the present invention uses a simple 3D VTI hierarchical model to verify the effectiveness of the strain field reconstruction method based on the spirally wound optical fiber DAS provided by the present invention.

[0120] Figure 5 FIG. 1 is a schematic diagram of a VTI layered model inserted into a vertical wellbore according to an embodiment of the present invention. Figure 5 As shown, the red explosion graphic represents a single source, the black vertical line represents the wellbore wrapped with spiral optical fiber, and the reflector is located at a depth of 1000m.

[0121] The model size is 2000×2000×2000 meters, and the spatial step is 10 meters. The horizontal reflector is located at a depth of 1000 meters. The upper layer parameters are v p =2500m / s,v s = 1800 m / s, ρ = 2.0, ε = 0.15, δ = 0.15 and γ = 0.1, the bottom layer has parameters v p =3000m / s,v s =2200m / s, ρ=2.2, ε=0.22, δ=0.20 and γ=0.15. The model is smoothed by 5×5 Gaussian filter. A 20Hz Ricker wavelet is used as the explosion source with a time sampling of 1 millisecond. A source point is deployed on the surface ( Figure 5 The red explosion point in the figure is located at (1000,1000) meters. In order to detect the seismic strain field signal, a wellbore is placed in the layered model ( Figure 5 ), its position ranges from (1000,500,0) to (1000,500,2000) meters.

[0122] Apply the design proposed by the present invention (such as Figure 4 As shown in the figure, a spiral optical fiber is deployed along the wellbore from 0 to 2000 meters. The gauge lengths are 1.25 meters (M=1) and 4.85 meters (M=5). According to the value of N (N=1), six 2π winding cycles are required for six sampling points. According to the above parameters, the straight-line distance of the six optical fiber winding cycles is about 3 meters, which is much smaller than the spatial step of the layered model. Therefore, 200 detectors can be placed at each grid point along the wellbore with a spatial step of 10 meters. Each data corresponding to the detector can be reconstructed from the DAS response records of six sampling points. The present invention first simulates an accurate strain rate record as a reference solution.

[0123] Figure 6 is a precision strain rate recording diagram of a VTI layered model with a length of 2 seconds provided in an embodiment of the present invention, Figure 7 is a strain rate record diagram of a VTI layered model reconstructed with a gauge length of 1.25 meters provided in accordance with an embodiment of the present invention, Figure 8 is a strain rate record diagram of a VTI layered model reconstructed with a gauge length of 4.85 meters provided in accordance with an embodiment of the present invention, Fig. 9 It is a comparison diagram of single-channel data extracted from original data and strain rate records reconstructed by DAS according to an embodiment of the present invention.

[0124] Specifically, the DAS response is generated by formula (12) and formula (18). Figure 7 and 8 The strain rate record is reconstructed by the spiral fiber solution proposed in the present invention (Formulas 13 and 18), with gauge lengths of 1.25 meters (M=1) and 4.85 meters (M=5). Figure 6 In comparison, these earthquake records show good recovery results. Figure 6 , Figure 7 and Figure 8 The single-channel data at Trace number = 50 was extracted for amplitude comparison ( Fig. 9 ). It can be seen that Fig. 9 The reconstructed strain rate data show good consistency with the original components, which proves the effectiveness of the method proposed in this invention.

[0125] In order to test the robustness of the technical solution of the present invention, a 3% random noise data amplitude was added to the DAS record of the layered model, and the noise was concentrated in the area with weak signals within the data frequency band.

[0126] Fig.10is a strain rate recording diagram of a VTI layered model with a DAS data amplitude of 3% random noise provided by an embodiment of the present invention, Fig.11 is a random noise map extracted from the difference between the reconstructed strain rate record and the original record of a VTI layered model provided by an embodiment of the present invention. Specifically, Fig.10 Strain rate records recovered from noisy DAS data are shown. These results are contaminated by random noise, but still retain the correct useful waveform information. In addition, the noise ( Fig.11 ) is from the original strain record ( Figure 6 ) and reconstructed strain records ( Fig.10 ) are extracted from the difference between . These pure noise data show that no useful information is lost in the reconstruction of the present invention ( Fig.10 ). However, the present invention only shows the recovered strain records in noisy data with a small gauge length (GL=1.25m). Due to the large condition number, the reconstruction results of large gauge lengths are not shown because they are almost completely covered by noise. Improving noise resistance is a direction that needs further research in the future.

[0127] Embodiment 3

[0128] Fig.12 Schematic diagram of a strain field reconstruction system based on a spirally wound optical fiber DAS according to an embodiment of the present invention, the system is applied to a spirally wound optical fiber distributed acoustic sensor. Fig.12 As shown, the system includes: a winding module 10, a setting module 20, a collection module 30 and a reconstruction module 40.

[0129] Specifically, the winding module 10 is used to periodically wind the detection optical fiber of the sensor on the wellbore of the target underground strain field; wherein, the detection optical fiber in each winding cycle includes a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle.

[0130] The setting module 20 is used to set two sampling points at a first angular sampling interval in the first optical fiber segment and the third optical fiber segment, and to set four sampling points at a second angular sampling interval in the second optical fiber segment and the fourth optical fiber segment.

[0131] The acquisition module 30 is used to obtain the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field.

[0132] The reconstruction module 40 is used to reconstruct the original strain component of the target underground strain field based on the DAS response of each sampling point.

[0133] Specifically, the reconstruction module 40 is further used for:

[0134] Based on the sampling point distribution of the sensor, a projection matrix of the mapping relationship between the DAS response of each sampling point and the original strain component of the target underground strain field is determined;

[0135] Based on the projection matrix and the DAS response of each sampling point, the original strain components are reconstructed.

[0136] The present invention further provides an electronic device, comprising: a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method provided in the embodiment of the present invention when executing the computer program.

[0137] The present invention also provides a computer-readable storage medium, which stores computer instructions. When the computer instructions are executed by a processor, the method provided in the embodiment of the present invention is implemented.

[0138] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0139] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A strain field reconstruction method based on spirally wound optical fiber DAS, characterized in that: Applied to a spirally wound optical fiber distributed acoustic sensor; the method comprises: The detection optical fiber of the sensor is periodically wound on the wellbore of the target underground strain field; wherein the detection optical fiber in each winding cycle includes a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle; Two sampling points are respectively set at a first angle sampling interval on the first optical fiber segment and the third optical fiber segment, and four sampling points are respectively set at a second angle sampling interval on the second optical fiber segment and the fourth optical fiber segment; Acquire a DAS response of each sampling point when the sensor detects a seismic signal of the target underground strain field; Based on the DAS response of each sampling point, the original strain component of the target underground strain field is reconstructed.

2. The method according to claim 1, characterized in that: The winding period of the detection optical fiber includes a 2π period, the first angle sampling interval includes π / 8, and the second angle sampling interval includes 3π / 16.

3. The method according to claim 1, characterized in that: Reconstructing the original strain component of the target underground strain field based on the DAS response of each sampling point includes: Determine, based on the sampling point distribution of the sensor, a projection matrix of a mapping relationship between a DAS response of each sampling point and an original strain component of the target underground strain field; The original strain component is reconstructed based on the projection matrix and the DAS response of each sampling point.

4. The method according to claim 3, characterized in that: The mapping relationship includes: ε=(G T G) ―1 G T e d Where ε=[ε xx ε yy ε zz ε yz ε xz ε xy ] T is the strain vector containing the original strain components, ε d for the DAS response; G is the projection matrix, which is a 6×6 matrix, and the i-th row vector is Wherein, i is the serial number of the sampling point, for the sampling points of the first optical fiber segment and the second optical fiber segment: For the sampling points of the third optical fiber segment and the fourth optical fiber segment: α1 is the first winding angle, α2 is the second winding angle, θ s1 is the first angle sampling interval, θ s2 is the second angle sampling interval, M is a preset non-negative integer, GL is the gauge length, and r is the winding radius of the wellbore.

5. A strain field reconstruction system based on spirally wound optical fiber DAS, characterized in that: Applied to spirally wound optical fiber distributed acoustic sensor; including: winding module, setting module, acquisition module and reconstruction module; wherein, The winding module is used to periodically wind the detection optical fiber of the sensor on the wellbore of the target underground strain field; wherein the detection optical fiber in each winding cycle includes a first optical fiber segment, a second optical fiber segment, a third optical fiber segment and a fourth optical fiber segment connected in sequence, the winding angle of the first optical fiber segment and the third optical fiber segment is a first winding angle, the winding angle of the second optical fiber segment and the fourth optical fiber segment is a second winding angle, and the first winding angle is not equal to the second winding angle; The setting module is used to set two sampling points at a first angle sampling interval on the first optical fiber segment and the third optical fiber segment, and to set four sampling points at a second angle sampling interval on the second optical fiber segment and the fourth optical fiber segment; The acquisition module is used to obtain the DAS response of each sampling point when the sensor detects the seismic signal of the target underground strain field; The reconstruction module is used to reconstruct the original strain component of the target underground strain field based on the DAS response of each sampling point.

6. The method according to claim 1, characterized in that: The reconstruction module is also used for: Determine, based on the sampling point distribution of the sensor, a projection matrix of a mapping relationship between a DAS response of each sampling point and an original strain component of the target underground strain field; The original strain component is reconstructed based on the projection matrix and the DAS response of each sampling point.

7. An electronic device, characterized in that: include: A memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the method according to any one of claims 1 to 4 when executing the computer program.

8. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and when the computer instructions are executed by a processor, the method according to any one of claims 1 to 4 is implemented.

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